The invention relates to a conductive pigment containing of two or more components and having particles of different shapes.
Electrically conductive pigments are today used in many sectors, for example for preparing antistatic coatings for electronic instruments, antistatic floor coverings, antistatic finishes for explosion-protected rooms or electrically conductive basecoats for the coating of plastics.
Currently carbon black and graphite are used for the preparation of dark conductive pigments and nickel-coated graphite, metal platelets and mixed oxides, such as, for example, antimony-doped tin dioxide, for the preparation of light-colored pigments. The mixed oxides mentioned can also be present as a coating on base materials, for example mica or barium sulphate.
However, owing to their high light absorption in the visible region of the spectrum, carbon black and graphite cannot be used for transparent or colored coatings. A further problem is the strong absorption in the IR region, which, for example on irradiation by sunlight, leads to heating of the coated articles which in many cases is undesirable. Metal platelets are susceptible to corrosion and can give rise to the formation of hydrogen in aqueous formulations.
The previously used inorganic oxides or mixed oxides in the form of platelets, for example using mica as the base material, or in a form in which their dimensions along the three directions of space are about the same, for example support-free antimony-doped tin oxide, must, due to their geometrical shape, be used in high concentrations in order to show a substantial effect.
It is known to increase the conductivity of pigments based on inorganic oxides by applying an intermediate layer between the metal oxide film and the conductive layer. In EP 0,373,575, conductive platelet-like pigments are described which comprise a platelet-like (i.e. platelet-shaped) substrate covered with a metal oxide film or of a platelet-like metal oxide. They are coated with an electrically conductive layer of antimony-doped tin dioxide, a silica layer being arranged between the metal oxide film and the conductive layer in order to increase the conductivity. The resistivity of these pigments is more than a power of ten less, and thus the electrical conductivity correspondingly higher, compared with comparison pigments without a silica intermediate layer.
However, application of an additional layer to the pigment constitutes a substantial additional expenditure during preparation, thus making the product more expensive. Furthermore, the conductivity values of the pigment do not meet high requirements.